1. Field of the Invention:
The invention relates to an improved method and apparatus for molding a dental prosthesis in situ in a patient's mouth.
2. Description of the Prior Art:
It is broadly old to form a temporary prosthesis according to method steps outlined below:
1. Make a negative impression of the patient's mouth prior to the preparation of the teeth to receive a prosthesis;
2. Prepare the teeth to receive the prosthesis by cutting down as necessary, blocking out the undercuts on the prepared teeth and lubricating the prepared teeth to prevent the prosthesis forming material from sticking to the teeth;
3. Mix a cold curing acrylic resin mixture into a dough-like consistency;
4. Pack the dough-like mix of acrylic resin into the negative impression in the area to be restored;
5. Reseat the negative impression with the mass of acrylic resin in the mouth over the prepared teeth, the objective being to form a temporary prosthesis, the inner surface of which is formed by the surfaces of the cut down teeth and the outer surface of which is formed by the inner surface of the negative impression;
6. Allow the acrylic resin to cure;
7. Remove the impression from the mouth leaving the cured acrylic over the teeth; and
8. Remove the cured acrylic from the mouth and trim off any excess to form a temporary restoration to be worn by the patient while a permanent restoration is being made.
The method described above is beset by a number of disadvantages which the present invention seeks to overcome. When the dough-like mass of acrylic if formed by seating the impression holding the acrylic in place in the mouth, the dough-like mass of acrylic, which is in excess of the amount required to form the prosthesis, squeezes out between the impression and the mouth forming a flash around the restoration. This flash flows over the adjacent teeth and tissue creating a large mass of material which has to be trimmed from the restoration before it is usable. The flash formed does not permit the impression to be fully seated in the mouth resulting in the restoration (i.e. prosthesis) being oversized. Further recontouring of the restoration to bring the restoration into proper occlusal relationship with the opposing teeth in the other arch is thus required.
The doughiness of the acrylic mass exerts hydraulic pressure against the contour of the flexible impression material, thus deforming the surface of the prosthesis. The resulting deformaties must also be corrected.
During the removal of the prosthesis with the attached flash from the mouth, the prosthesis is warped to a certain degree. Also, during the trimming involved in removing the flash and recontouring the prosthesis to its proper shape, the prothesis is further distorted by handling and by the heat generated by the grinding of the acrylic resin. These deformities result in a prosthesis which does not fit back on the tooth stumps in an ideal manner and the poor fit of the prosthesis over the tooth stumps can lead to one or more of the following conditions:
1. Breakage of the prosthesis under mouth stresses created by the poor seating of the prothesis on the tooth stumps;
2. The rocking loose and coming out of the prothesis when subjected to mouth stresses.
3. Orthodontic movement of the teeth during mouth stress, resulting in a movable abutment tooth, in which case, a permanent bridge, when completed, would not seat properly and it would be necessary to remake the permanent bridge.